This invention relates to nuclear spin tomography or nuclear resonance tomography, in general and more particularly, to a gradient coil system for use in a nuclear spin tomography installation.
A gradient coil system for a nuclear spin tomography installation which surrounds a hollow cylindrical support body, the cylindrical axis of which extends in the z direction of an orthogonal x-y-z coordinate system with the coordinate origin in the center of an imaging region, in which direction the magnetic field B.sub.z of a base field magnet comprising several field coils is also oriented, which gradient coil system contains, for generating a largerly constant field gradient G.sub.z =.differential.B.sub.z /.differential.z, at least two annular individual coils which are at least approximately symmetrical to the x-y plane extending through the center of the imaging range, and further, at least one set of pairs of saddle-shaped individual coils which are arranged at least approximately symmetrically with respect to this symmetry plane, for each of the x and y axes, and which are provided for generating field gradients G.sub.x =.differential.B.sub.z /.differential.x, in the x direction largely constant in the imaging range or corresponding field gradients G.sub.y =.differential.B.sub.z /.differential.y, in the y direction is described in the published European Patent Application EP No. 56 691 Al.
In the field of medical diagnostics, image producing methods have been developed, in which, through computer or measurement analysis of integral resonance signals of nuclei such as protons, an image similar to an X-ray tomogram can be constructed from the spatial spin density and/or relaxation time distribution of a body to be examined, especially a human body. The corresponding method is called nuclear spin tomography, nuclear magnetic resonance tomography, or also zeugmatography ("Nature", vol. 242, 1973, pages 190 and 191).
For nuclear spin tomography (nuclear magnetic resonance tomography) installations, a strong stationary base field, for instance, of the order B.sub.0 =1.5 T is desired, on the intensity of which the magnitude of the resonance signal depends and which must meet stringent requirements with respect to its homogeneity. Thus, a suitable base field magnet which generally consists of several coil sections must have a field deviation of less than 50 ppm in a spherical volume with a diameter of about 50 cm.
This magnetic base field will be assumed to be oriented, for instance, in the z direction of an orthogonal x-y-z coordinate system, where the z axis is the examination axis, along which the body to be examined is placed in the magnetic field. The coordinate origin is to be placed in the center of the imaging or examination area.
A high-frequency coil arrangement for the corresponding precession frequency of the nuclear spins to be considered in order to excite these spins and, optionally also receive the induction signals, must also be provided.
Finally, a system of gradient coils is required which operate set of supplemental fields G.sub.z =.differential.B.sub.z /.differential.z, G.sub.x =.differential.B.sub.z /.differential.x and G.sub.y =.differential.B.sub.z /.differential.y. These supplemental fields are small as compared to the base field B.sub.z oriented in the z direction. The gradient fields, switched on in a predetermined order, permit a distinction as to the location, by the course of the precession frequency of the nuclei versus the location (see, for instance, "Journal of Magnetic Resonance", vol. 18, 1975, pages 69 to 83 or vol. 29, 1978, pages 355 to 373).
In general, the mentioned field gradients G.sub.x, G.sub.y and G.sub.z can be generated by magnetic quadrupoles. In the case of installations for nuclear spin tomography, it is necessary to take into consideration that these coils must be arranged in the interior of the base field magnet, and sufficient space must remain empty for supporting the body to be examined.
The corresponding gradient coils of the nuclear spin tomography installation known from the cited European application are rigidly connected to a hollow cylindrical support body which can be inserted into the base field magnet; the support axis coincides here with the magnet axis and points in the z direction of an orthogonal x-y-z coordinate system. The z gradient G.sub.z is generated by two ring coils which are arranged symmetrically to an x-y plane which is oriented perpendicularly with respect to the cylinder axis and goes through the center of the imagining area. For generating the x gradient, G.sub.x, two pairs of saddle-shaped coils are provided, the coils of each pair being put in place in a diametrical position on the outer shell of the hollow cylindrical support body. For the y gradient, G.sub.y, a corresponding system of four saddle coils is provided which are arranged shifted on the support body by 90 degrees in the circumferential direction relative to the x gradient coils. The two pairs of individual coils of each set of coils are arranged symmetrically on both sides of the mentioned x-y plane. Besides a direct fastening of the individual gradient coils on the hollow cylindrical support body, a separate mounting tube can optionally also be used, on which the individual coils are placed. The mounting tube itself is then supported on the inner hollow cylindrical support body which is substantially longer.
The form, subdivision and arrangement of the individual G.sub.x and G.sub.y saddle coils and the G.sub.z ring coils are given. If they are operated in the strong base field, socalled Lorentz forces occur, which engage the coils as distributed loads. These Lorentz forces are variable due to typical switching sequences of the individual gradient fields in the coils, so that these coils, themselves, are excited to force vibrations. Through solid-borne and air-borne sound, the picked up energy is transferred from the coil system, and possibly, via the separate mounting tube to the hollow cylindrical support tube which acts somewhat as a loudspeaker diaphragm, and thereby fills the space containing the body to be examined with sound waves, the frequencies of which are mainly in the range between 200 Hz and 1000 Hz. Thus, it was possible, for instance, to measure, in the center of the useful volume, sound levels of up to 95 dB(A) which are insufferably high for a patient to be examined. For limiting the radiated power, the gradient coils can be operated with a smaller nominal current; this, however, leads to a corresponding reduction of the image quality of the nuclear spin tomography installation.
It is, thus, an object of the present invention to develop the gradient coil system mentioned at the outset in such a way that a reduction of the sound level caused by it in the hollow cylindrical support body is achieved without having to tolerate, therewith, a reduction of the image quality.